The present invention is directed to emulsion adhesive compositions used in wet laminating operations to produce multi-ply substrates which may be used to fabricate bags or other products suitable for use in microwave ovens.
Microwavable bags, particularly those adapted for the microwave cooking of popcorn, are generally formed from a laminate comprising two layers of flexible microwave-transparent sheet material. Generally the outer layer is bleached kraft paper and the inner layer, or liner, treated or untreated Mylar or polyethylene. The liner should be a grease proof, heat sealable flexible sheet in order to prevent oil absorption by the paper during storage and popping.
In a typical construction, the bag is formed of a single rectangular piece of the laminate which is subjected to a series of folding and heat sealing operations so as to form a pleated or expandable bag. After filling of the bag with the popcorn, shortening and seasonings, a final heat seal is often used to bond the top portion of the bag. The final bag is gusseted so as to be flexible and expandable during heating. The package has no openings or vents so that the steam given off while heating will expand the bag. It is important that the bag be not only flexible and expandable under stream pressure but also be leak proof during the cooking process. Typical of such constructions are those disclosed, for example, in U.S. Pat. Nos. 4,292,332, 4,461,031 and 4,571,337.
These stringent requirements place heavy demands on the adhesive used to laminate the two plies of sheeting. One of the most difficult tests for the adhesive compositions to pass is in the heat sealing operation where the seams are subjected to temperatures in the order of about 175.degree. C. to 300.degree. C. At these temperatures, the lining becomes thermoplastic and heat seals to itself to form the seam; however, many of the adhesives utilized in laminating the two plies soften at these temperatures causing the bond between the two plies or substrates to fail, weakening the bag, often causing it to delaminate and/or rupture during subsequent use in the microwave oven.
In order to produce adequate initial bonds together with the need to meet the high temperature requirements, laminating adhesives for use in microwaveable products have heretofore been prepared by blending various polymers such as blends of ethylene vinyl acetate emulsion, vinyl acetate emulsion and a soft ethylene vinyl acetate-acrylic latex and especially by adding thereto substantial amounts of epoxy components, often in conjunction with cross-linking activators therefor. These compositions are often unstable over a period of time and suffer the additional disadvantages of short open time and incompatibility with other emulsions.